1. Field of the Invention
The invention relates to a method and a device for the distribution of fuel in a burner for gas turbines and heating boilers which is suitable for both liquid and gaseous fuels and has an airblast nozzle.
2. Discussion of Background
A plurality of airblast nozzles are disclosed by Lefebvre "Airblast atomization", Prog. Energy Combust. Sci. Vol. 6, p. 239 ff. They each consist of a liquid-fuel line, an annular airblast line and a prefilming tube.
The liquid-fuel line is either directed radially into the airblast nozzle (ibid, FIG. 7) or is arranged axially in the fuel lance, i.e. inside the airblast line (ibid, FIG. 5). Here, it can be connected to the prefilming tube directly or via distribution lines. The prefilming tube consists of a swirl generator having an integrated settling chamber, a weir, a prefilming lip, and a central nozzle pin arranged as counterpart to the prefilming lip. The airblast line is subdivided by the prefilming tube into an inner and an outer line in each case.
During operation with liquid fuel, this fuel is passed from the feed line into the settling chamber and from there via the weir onto the prefilming lip, where a film of liquid fuel forms. This film of liquid fuel is atomized at the tip of the prefilming lip by means of blast air from the inner and outer airblast line and the resulting fuel drops are injected into the inner space of the burner.
Lefebvre shows, inter alia, the following possibilities of obtaining good atomization, i.e. of forming relatively small fuel droplets:
a) An optimum quantity ratio of atomization air to liquid fuel of 4:1 to 5:1 (ibid, FIG. 15); therefore the atomization quality deteriorates below a quantity ratio of 4:1, whereas only small improvements in the atomization can be achieved above a quantity ratio of 5:1 by feeding larger air quantities. However, if this quantity ratio drops below 2:1, according to Lefebvre a considerable impairment in the atomization quality can be found. PA1 b) Maximum physical contact of the atomization air with the liquid fuel; therefore the prefilming angle and thus the spray angle of the airblast nozzles are made relatively large at about 45.degree. to 60.degree. (ibid, FIG. 6, FIG. 7). However, this requires a relatively intensive swirl of the fuel. PA1 c) As high a velocity as possible of the atomization air sweeping past on both sides of the prefilming lip (ibid, FIG. 15); here, higher velocities of the atomization air are not only able to provide for a better atomization quality but they are also said to prevent liquid fuel from striking the inner surface of the burner or the airblast nozzle.
Increased fuel velocities occur at the weir edge of such an airblast nozzle transversely to the prefilmer. Consequently, the separation of fuel droplets and/or the forming of a relatively thick film of liquid fuel already occurs at this point. Both effects counteract the development of small fuel droplets and thus adversely affect the combustion. In addition, the fuel drops can strike the nozzle wall and thus increase the risk of carbonization. In the prior art cited, these disadvantages are countered through the use of a large air quantity in relation to the fuel quantity. However, such a large air charge in the airblast nozzle is very unfavorable during operation with gaseous fuel, since this destabilizes the gas flame and greatly reduces its lean extinction limit. The known airblast nozzles are of small dimensions in the area in front of the weir edge and are thus susceptible to carbonization. On account of their large spray angle, portions of the liquid fuel can reach the inner surface of the burner and cause overheating there. In addition, the atomization quality is impaired.